1. Field of the Invention
This invention relates to switches for switching very large currents and has particular application to switches for switching very large DC currents into parallel rails such as are used in the electromagnetic propulsion of projectiles.
2. Description of the Prior Art
In switching very large DC currents, on the order of several hundred thousand amperes, the current interruption or commutation is not instantaneous resulting in the generation of very large arc. In order to maintain themselves, these arcs require a gaseous material and will vaporize anything available, including the switch contacts, to generate the necessary conducing medium. Without proper design, these arcs can cause very considerable damage to the switching device, thereby significantly reducing its useful life.
One application where very high DC currents must be switched is in electromagnetic propulsion of projectiles such as devices commonly known as parallel rail launchers or rail guns. In the rail gun, a projectile is placed in the gap between two conductive rails rigidly fixed in spaced parallel relationship to each other. When a very high DC current is injected into the fixed rails, the electromagnetic field generated drives the projectile down the rails and out of the end of the gun at very high velocities, which can be in the order of several thousand meters per second.
Presently, the very high DC currents are commutated into the rail gun by a rail switch, which is in effect a second rail gun, one rail of which has a first portion connected to one rail of the gun at the breech and a second portion electrically isolated from the first and connected to the breech end of the second gun rail. The switch armature is in the form of an electrically conductive projectile which spans the gap between the switch rails. The armature is restrained at the energized end of the switch rails, but when released is driven down the switch rails at high velocity by the electromagnetic field generated by the current to be switched. As the armature passes from the first to the second portion of the switch rail connected to the gun rails, the current is commutated into the gun rails. Typically, the armature of the rail switch is made up of wafers of conductive material bent so as to supply a spring force against the switch rails for better sliding electrical contact. Early work on such a rail switch is discussed in the thesis of J. P. Barber entitled "The Acceleration of Macroparticles and a Hypervelocity Electromagnetic Accelerator" published by the Australian National University (ANU), March 1972. A modified form of such a rail switch is disclosed in my copending, commonly owned U.S. patent application Ser. No. 100,302 filed Dec. 4, 1979 now U.S. Pat. No. 4,369,692. These rail switches provide for rapid commutation of the current to the gun rails which reduces switch damage due to arcing.
In the ANU rail switch, an insulating insert is provided in the gap between the first and second portions of the switch rail connected to the rail gun in order to start the arc that is essentially unavoidably present during commutation. In my modification of the basic rail switch, an insulating insert is also provided in the other switch rail adjacent the first insert so that two arcs are generated in series as the armature passes over the inserts. This doubles the total arc voltage resulting in a more rapidly completed commutation of the current. My modified rail switch further extends the life of the rail switch by providing an improved sliding armature and by providing arc resistant inserts in the switch rails at their juncture with the resistive inserts where the arcs are struck.
After the current has been commutated to the gun rails, the high speed armature of the rail switch, which can easily reach a speed of 50 meters per second, must be stopped, returned to the starting position and restrained in preparation for the next firing. This is a limitation in rapid firing of the rail gun. In my copending application identified above, I have proposed a rail switch in which the rails form a circular path for the armature. While this shortens the distance that the armature must be moved for resetting, it still requires stopping the armature, repositioning it and restraining it.
In addition to the limitations of the rail switches in rapid firing, they are bulky and expensive. They require rails several meters long which must be precisely and rigidly held in place to resist the substantial forces tending to spread them apart, and the armatures have a limited life.
As an alternative to passing a large current through an electrical conductive projectile and accelerating the projectile through the magnetic forces created thereby, the projectile can be driven by an arc created in the rail gun behind the projectile. These prior art arc driving systems, such as those shown in my copending applications Ser. No. 116,118 filed Jan. 28, 1980 now U.S. Pat. No. 4,319,168 and Ser. No. 137,059 filed on Apr. 3, 1980 now U.S. Pat. No. 3,347,463 use a rail switch for commutating current into the rail gun and since the arcing which effects commutation of the current occurs in the switch, not the gun, a fuse type device at the rear of the projectile is required to initiate the desired propelling arc. The large current flowing through the fuse causes it to explode, thereby striking or initiating the arc or plasma which drives the projectile through the rail gun. Again, such an arrangement is quite suitable for single firings, but with rapid burst firing, the debris from many fuse explosions is likely to both impede fuse contact in subsequent firings and to finally restrict the rail bore to such an extent as to damage the projectile sealing faces. This, in turn, can lead to hot gases bypassing the projectile with the arc eventually leapfrogging the projectile and eliminating the projectile driving force.